This explains a lot doesn't it?
So why is it so hard to build a great bullpen?
The simple answer is because relievers, more than any other specific type of player, are alarmingly inconsistent from year to year.
Indeed, a glance at a list of the top 25 relievers in the majors last season, based on opponents' batting average (a better gauge of a reliever's effectiveness than ERA), shows how fleeting bullpen success can be. Only four pitchers from that list are in the top 25 again this season (through Thursday's games).